1. Field of the Invention
The present invention relates to a picture display system which converts a video signal in the high definition television system (HDTV) into a video signal in a conventional television system to display the converted video signal on the screen of the conventional television system. More particularly, the present invention relates to a system in which part of a picture of the high definition television system is sampled to be displayed on the conventional television screen without "thinning out" the number of scanning lines in the high definition television picture.
2. Prior Art
A high definition television system, for example, has been proposed as a new television system that can replace the conventional television system not only for improved picture quality and sound quality, but also for visual and psychological effects such as presence and impression. Hi-Vision is one such high definition television system and the MUSE system has been proposed as a transmission system for the Hi-Vision system.
The MUSE system differs from the conventional television systems such as the NTSC system in the picture system, scanning system and sound system. Therefore, the MUSE signal cannot be directly displayed on the TV screen of the conventional television system. A MUSE-to-NTSC converter (called a down converter) has been proposed for overcoming the differences in the number of scanning lines, aspect ratio, and line frequency to convert the MUSE signal into the NTSC signal for a suitable display.
FIG. 4 illustrates the conversion of output of the down converter into the NTSC picture. The active scanning lines per frame of 1032 in the MUSE signal is reduced to one-third thereof, i.e., 344 by picking up one line for every three lines to thereby convert the high definition picture into the NTSC picture with the aspect ratio of 16:9 in the MUSE system being unchanged. The number of samples in each of the scanning lines is also reduced from 374 samples in the HDTV system to 132 samples in the NTSC system. In this system, since the aspect ratio of the NTSC picture is 4:3 (i.e., 16:12), an area of about 30% including the upper and lower portions of the NTSC picture will be left unused though the horizontal dimension of the picture will remain the same as that of the MUSE system. Each of the 344 scanning lines is displayed with the same scanning density as in the NTSC system which is less than the scanning density in the MUSE system. Thus, this prior art system suffers from a drawback that because the number of scanning lines is reduced from 1032 to 344, the picture displayed on the screen of the NTSC system no longer has the advantage of being high definition, in fact, the picture quality is less than that in the NTSC system.
Japanese Patent Application No. 63-92987 discloses a system in which the high quality picture can be displayed on the conventional television receiver with the high grade of definition in the high definition television picture being unimpaired. In this prior art system, the Hi-Vision picture is divided into a plurality of pictures, each of which is converted into the NTSC signal to form a single picture as a whole which can be displayed using a plurality of NTSC receivers. While the system is capable of displaying a picture having a high degree of definition with practically sufficient brightness on the conventional NTSC receiver by using a projection unit for multi-display purpose, complex signal processing is necessary to convert the scanning lines from Hi-Vision to NTSC. As shown in FIG. 5, the Hi-Vision picture is divided into three sections vertically and into four sections horizontally. Each of the 12 pictures is displayed on an NTSC receiver and the 12 pictures are properly arranged to form a single large display as a whole. The NTSC system is based on 525 horizontal scanning lines. Thus, the 1575 (=525 times 3) horizontal scanning lines are required to display the vertical portion using three NTSC televisions. Since the Hi-Vision is based on 1125 scanning lines, a scanning line conversion process is required where seven horizontal scanning lines are extracted from every five horizontal scanning lines. This conversion process necessitates the complexity of circuit design which leads to a large and expensive apparatus.